1. Field of the Invention
The present invention relates to downhole apparatus and particularly, but not exclusively, to multi-cycle circulating subs used during downhole drilling operations.
2. The Prior Art
It is often necessary in downhole drilling operations to bypass or partially bypass the flow of wellbore fluid down the drill string into the wellbore annulus. For example, this may be necessary where the desired fluid flow rate to drive a drilling tool is insufficient to carry all the drilled material up the annulus to the surface. In these circumstances, a circulating sub may be used to allow the flow rate required to remove the drilled material to be pumped into the annulus whilst maintaining the lower flow rate required at the drilling tool.
It is known to provide a circulating sub with an axially movable piston for opening and closing vent apertures. The vent apertures are provided in a body of the sub and allow wellbore fluid pumped downhole through a central bore of the sub to pass into the surrounding wellbore annulus. Opening and closing of the vent apertures by means of the piston is controlled by a pin and groove arrangement. The pin is located in one of the piston and body and is received within the groove provided in the other of the piston and body. The profile of the groove is such that axial movement of the piston results in rotation of the piston within the body. Furthermore, the extent of axial piston movement is limited by the groove profile. Thus, the piston may be moved axially downhole by means of a predetermined fluid flow rate and returned uphole by means of a biasing spring so as to cycle the piston into a position wherein the control groove permits subsequent movement of the piston from a vent aperture closed position to a vent aperture open position.
A problem associated with the aforementioned prior art means for controlling the piston is that there can be a tendency for the control pin to become damaged within the control groove as a result of axial and rotational forces acting on the piston. These forces can shear the control pin within the control groove.
In addressing this problem, our UK patent application number 0116472.2 provides apparatus comprising a piston slidably mounted in a body between positions in which at least one aperture in the body is opened and closed. Movement of the piston is controlled by one or more pins (secured to one of the body and a control member) and a control groove (formed in the other of the body and control member) in which a portion of the or each pin is received. An arrangement of elements respectively connected to the control member and body is such that, as the control member moves axially, lengths of said elements locate adjacent one another so as to provide resistance to relative rotation in at least one direction of the control member and body. The relative rotation is a rotation which presses the control member against the control groove. The elements are also arranged to limit axial movement of the control member. The apparatus thereby provides means by which the risk of damage to the control pin is reduced.
The present invention provides apparatus for selectively providing fluid communication between the interior of a downhole assembly and the exterior thereof said apparatus comprising: a body incorporating a wall provided with at least one aperture extending therethrough; a piston having a longitudinal bore extending therethrough and being slidably mounted in the body so as to be movable between a first position relative to the body preventing fluid communication between the bore of the piston and the exterior of the body via the or each aperture and a second position relative to the body permitting fluid communication between the bore of the piston and the exterior of the body via the or each aperture; and controlling means for controlling the movement of the piston between the first and second positions, the controlling means comprising: a control member slidable in the body and movable by fluid pressure in the body in a first axial direction relative to the body; a spring biasing the control member in an opposite axial direction of the body; a pin secured to one of the bodes and the control member; and a control groove in which a portion of the pin is received formed in the other of the body and the control member, the control groove being shaped to limit axial displacement of the control member generated by pressure variations in the body such that only after a predetermined number of movements of the control member to a first axial position is the control member able to move to a second axial position so as to displace the piston from one of the first and second piston positions to the other of the first and second piston positions; wherein the controlling means further comprises a first element connected to the control member so as to prevent relative rotation between the first element and the control member, and a second element connected to the body so as to prevent relative rotation between the second element and the body, wherein the arrangement of said element is such that, in the first axial position of the control member, the first and second elements normally abut one another so as to resist axial movement of the control member toward the second axial position, said elements locating offset relative to one another so as to allow movement of the control member to the second axial position only after a predetermined number of movements of the control member to the first axial position; and wherein the spring is located in a chamber defined between the control member and the body, and at least one vent opening is provided in the body for venting fluid located in the chamber to the exterior of the body. The arrangement of said elements may be such that, as the control member moves from said first axial position to said second axial position, increasing lengths of said elements locate adjacent one another so as to provide resistance to relative rotation, in at least one direction, of the control member and body, said relative rotation being relative rotation which presses the control pin against the control groove.
Thus, in apparatus according to the present invention, movement of the control member past the first axial position is normally prevented by an abutment of the first and second elements and, as a consequence, an undesirable application of axial pressure by the control groove on the control pin may be avoided. Also, as the control member moves from the first axial position to the second axial position and thereby displaces the piston into one of the first and second piston positions, elements connected to the control member and apparatus body locate adjacent one another so as to provide resistance to relative rotation of the control member and body. As a consequence, relative rotation which tends to press a control pin against the control groove can be resisted and damage to the control pin thereby avoided. The first and second elements may be arranged so as to allow relative rotation between the control member and body as may be permitted by the control groove profile. However, the elements do not allow rotation which will press the control pin and groove against each other to the extent that damage to the pin may occur. Furthermore, as the control member is moved from said first axial position to said second axial position, the elements locate adjacent one another to an increasing extent by virtue of said elements sliding over one another in a collapsing telescoping type of movement. Thus, as the control member moves towards the second axial position, the elements are better able to resist relative rotation due to the increasingly long lengths of element portions located adjacent one another. Also, since the spring chamber may be exposed to wellbore fluid pressure, a resultant fluid pressure may be applied to the control member which, in use, reduces the risk of an accidental cycling of the control pin within the control groove.
Ideally, at least one vent opening is provided in the control member for venting fluid located in the chamber to the exterior of the body. The or each vent opening in the control member or the body may also be occluded so as to prevent a passage of fluid therethrough. The or each occluded vent opening may be occluded with a removable plug. Thus, the spring chamber can be vented to the piston bore or wellbore annulus depending on which set of vent openings are occluded.
It is also desirable for the axial movement of the piston to be limited by one or more stop shoulders provided on the body. A first shoulder may limit axial movement of the piston in a first direction. A second shoulder may limit axial movement of the piston in a second direction opposite to said first direction. In this way, the application of axial thrust forces to the or each pin with the piston in the uppermost and lowermost positions may be avoided.
It is preferable for said first element to remain axially spaced from said second element until the control member is axially moved to the first axial position. The arrangement of the first and second elements may be such that said elements become offset to one another, so as to permit axial movement of said elements past one another, only after said predetermined number of movements of the control member to the first axial position. The said elements may be offset angularly. It is also preferable for the arrangement of the first and second elements to be such that, when said elements are offset so as to permit their axial movement past one another, the control pin is received in one of a plurality of portions of control groove allowing the control member to move to the second axial position. The arrangement of the first and second elements may also be such that, when said elements are offset so as to permit their axial movement past one another, the control pin is received in a portion of control groove allowing the control member either to displace the piston in said first axial direction from the first piston position to the second piston position and then to a third piston position preventing fluid communication between the bore of the piston and the exterior of the body via the or each aperture, or to displace the piston in said first axial direction from the second piston position to the first piston position and then to a third piston position permitting fluid communication between the bore of the piston and the exterior of the body via the or each aperture.
The control groove may comprise a plurality of said portions allowing displacement of the piston to said third piston position. Movement of the control member in said first axial direction past the second axial position may be prevented by means of an abutment of the second element with the control member or a component connected thereto. The second element may also be releasably connected to the body. The second element may be releasably connected to the body by means of a shear pin. When in the second piston position, the piston may be located so as to seal a fluid pathway through the apparatus and thereby, in use, direct fluid flowing into said apparatus through the or each aperture. Also, the or each aperture may be arranged so that wellbore fluid flowing in use through the or each aperture from the interior of the apparatus is directed in a direction having a component parallel to the longitudinal axis of the apparatus.
Embodiments of the present invention will now be described with reference to the accompanying drawing.